Ventilatory control of the ‘isocapnic buffering’ region in rapidly-incremental exercise

doi:10.1016/0034-5687(89)90076-5

Respiration Physiology

Volume 76, Issue 3, June 1989, Pages 357-367

Respiration Physiolo...

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Abstract

During incremental exercise P_CO₂ does not fall for several work rates (‘isocapnic buffering’) above the anaerobic threshold (θ_an). We explored this apparent lack of compensatory hyperventilation in 24 normal subjects who underwent incremental cycling (15 W/min) to exhaustion. Ventilation, pulmonary gas exchange, and end-tidal gas tensions were computed breath-by-breath. In 10 subjects, arterial blood was sampled every 2 min throughout the test. Our findings confirmed the ‘isocapnic’ supra-θ_an region, but it consistently followed a progressive increase of PET_CO₂ in the sub-θ_an region. A similar pattern was evident for Pa_CO₂. The leveling-out of Pet_CO₂ and Pa_CO₂ was a result of breathing frequency increasing at θ_an, thereby shortening expiratory time, i.e., progressively truncating the continued increase in the alveolar P_CO₂ slope. Consequently ‘isocapnic buffering’ during incremental exercise does not reflect P_CO₂ which continues to be regulated at a constant sub-θ_an value. Rather it reflects a ventilatory response to the metabolic acidosis which levels a systematically-rising phase of Pet_CO₂ and Pa_CO₂, largely through a change in breathing pattern. Respiratory compensation, as reflected by a declining Pet_CO₂ and Pa_CO₂, does not occur typically for a subsequent 2 or more minutes.

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Cited by (153)

Do Clinical Exercise Tests Permit Exercise Threshold Identification in Patients Referred to Cardiac Rehabilitation?
2023, Canadian Journal of Cardiology
To evaluate the feasibility of “threshold-based” aerobic exercise prescription in cardiovascular disease, we aimed to quantify the proportion of patients whose clinical cardiopulmonary exercise test (CPET) permit identification of estimated lactate threshold (θ_LT) and respiratory compensation point (RCP) and to characterize the variability at which these thresholds occur.
Breath-by-breath CPET data of 1102 patients (65 ± 12 years) referred to cardiac rehabilitation were analyzed to identify peak O₂ uptake ( $\dot{V}$ O_2peak; mL·min^-1 and mL·kg^-1·min^-1) and θ_LT and RCP (reported as $\dot{V}$ O₂, % $\dot{V}$ O_2peak, and %peak heart rate [%HR_peak]). Patients were grouped by the presence or absence of thresholds: group 0: neither θ_LT nor RCP; group 1: θ_LT only; and group 2: both θ_LT and RCP.
Mean $\dot{V}$ O_2peak was 1523 ± 627 mL·min^-1 (range: 315-3789 mL·min^-1) or 18.0 ± 6.5 mL·kg^-1·min^-1 (5.2-46.5 mL·kg^-1·min^-1) and HR_peak was 123 ± 24 beats per minute (bpm) (52 bpm-207 bpm). There were 556 patients (50%) in group 0, 196 (18%) in group 1, and 350 (32%) in group 2. In group 1, mean θ_LT was 1240 ± 410 mL·min^-1 (580-2560 mL·min^-1), 75% ± 8% $\dot{V}$ O_2peak (52%-92% $\dot{V}$ O_2peak), or 84% ± 6%HR_peak (64%-96%HR_peak). In group 2, θ_LT was 1390 ± 360 mL·min^-1 (640-2430 mL·min^-1), 70% ± 8% $\dot{V}$ O_2peak (41%-88% $\dot{V}$ O_2peak), or 78% ± 7%HR_peak (52%-96%HR_peak), and RCP was 1680 ± 440 mL·min^-1 (730-3090 mL·min^-1), 84% ± 7% $\dot{V}$ O_2peak (54%-99% $\dot{V}$ O_2peak), or 87% ± 6%HR_peak (59%-99%HR_peak). Compared with group 1, θ_LT in group 2 occurred at a higher $\dot{V}$ O₂ but lower % $\dot{V}$ O_2peak and %HR_peak (P < 0.05).
Only 32% of CPETs exhibited both θ_LT and RCP despite flexibility in protocol options. Commonly used step-based protocols are suboptimal for “threshold-based” exercise prescription.
Afin d’évaluer la faisabilité de la prescription d’exercices aérobiques « fondés sur lessd'intensités esd en présence d’une maladie cardiovasculaire, nous avons voulu quantifier le pourcentage de patients dont l’épreuve d’effort cardiopulmonaire (CPET, de l'anglais cardiopulmonary exercise test) permet d'identifier le seuil lactique estimé (θ_LT) et le point de compensation respiratoire (RCP, de l'anglais respiratory compensatin point) ainsi que de caractériser la variabilité de ces seuils.
Les résultats des CPET respirations par respiration de 1102 patients (âge moyen de 65 ± 12 ans) orientés en réadaptation cardiaque ont été analysés afin de recenser la consommation pic d’oxygène ( $\dot{V}$ O₂_pic; ml·min⁻¹ et ml·kg⁻¹·min⁻¹) et ainsi que le θ_LT et le RCP (comme suit : $\dot{V}$ O₂, % $\dot{V}$ O₂_pic et pourcentage de la fréquence cardiaque pic [%FC_pic]). Les patients ont été regroupés en fonction de la présence ou de l’absence des seuils, soit groupe 0 : ni θ_LT ni RCP; groupe 1 : seulement θ_LT et groupe 2 : à la fois θ_LT et RCP.
La $\dot{V}$ O₂_pic moyenne était de 1523 ± 627 ml min⁻¹ (min.-max. : 315-3789 ml·min⁻¹) ou 18,0 ± 6,5 ml·kg⁻¹·min⁻¹ (5,2-46,5 ml·kg⁻¹·min⁻¹) et la FC_pic était de 123 ± 24 battements par minute (bpm) (52-207 bpm). Il y avait 556 patients (50 %) dans le groupe 0, 196 (18 %) dans le groupe 1 et 350 (32 %) dans le groupe 2. Dans le groupe 1, le θ_LT moyen était de 1240 ± 410 ml·min⁻¹ (580-2560 ml·min⁻¹), 75 % ± 8 % $\dot{V}$ O₂_pic (52-92 % $\dot{V}$ O₂_pic) ou 84 % ± 6 %FC (64-96 %FC_pic). Dans le groupe 2, le θ_LT était de 1390 ± 360 ml·min⁻¹ (640-2430 ml·min⁻¹), 70 % ± 8 % $\dot{V}$ O₂_pic (41-88 % $\dot{V}$ O₂_pic) ou 78 % ± 7 %FC_pic (52-96 %FC_pic) et le RCP était de 1680 ± 440 ml·min⁻¹ (730-3090 ml·min⁻¹), 84 % ± 7 % $\dot{V}$ O₂_pic (54-99 % $\dot{V}$ O₂_pic_max) ou 87 % ± 6 %FC_pic (59-99 %FC_pic). Comparativement au groupe 1, le θ_LT dans le groupe 2 a été atteint à une $\dot{V}$ O₂ plus élevée, mais à des % $\dot{V}$ O₂_pic et %FC_pic inférieurs (p < 0,05).
Seulement 32 % des CPET présentaient à la fois le θ_LT et le RCP, malgré la flexibilité des options du protocole. Les protocoles par paliers couramment utilisés sont sous-optimaux pour la prescription d’exercices fondés sur les seuils.
Reproducibility of NIRS-derived mitochondrial oxidative capacity in highly active older adults
2023, Experimental Gerontology
In-vivo techniques using near-infrared spectroscopy (NIRS) have been developed to assess skeletal muscle mitochondrial oxidative capacity. However, the test-retest and day-to-day reliability of NIRS-derived mitochondrial oxidative capacity has yet to be established in older individuals. Therefore, the primary aim of this study was to determine the day-to-day and test-retest reliability of NIRS-derived mitochondrial oxidative capacity in older adults. The secondary aim was to examine the relationship between NIRS-derived mitochondrial capacity and whole-body aerobic fitness.
Twenty-four healthy individuals (19 M, 5F; aged 60 ± 4 years; maximal oxygen uptake (V̇O_2peak) = 41.2 ± 6.8 ml.kg⁻¹.min⁻¹) completed three visits to the laboratory. Visit one assessed isometric maximal voluntary contractions of the knee extensors and aerobic capacity through an incremental exercise test. In visits two and three participants completed two measurements of NIRS-derived mitochondrial oxidative capacity in the vastus lateralis (VL).
NIRS-derived mitochondrial oxidative capacity was found to have good to excellent day-to-day reliability (Day 1 vs Day 2; coefficient of variation (CV) = 7.0 %; standard error of measurement (SEM) = 5.2; intra-class correlation coefficient (ICC) = 0.94) and test re-test reliability (Day 1 [Test 1 vs Test 2]; CV = 5.0 %; SEM = 3.7; ICC 0.97 and Day 2 [Test 1 vs Test 2]; CV = 6.3 %; SEM = 4.9; ICC = 0.93). NIRS-derived mitochondrial oxidative capacity was found to be significantly correlated with V̇O_2peak (r = −0.61; R² = 0.37; P = 0.002), oxygen uptake at the gas exchange threshold (r = −0.49; R² = 0.24; P = 0.02), and oxygen uptake at the respiratory compensation point (r = −0.57; R² = 0.32; P = 0.004).
NIRS provides a reliable method for deriving a measure of VL mitochondrial oxidative capacity in highly active older adults and demonstrates a significant relationship with measures of whole-body aerobic fitness.
Pick Your Threshold: A Comparison Among Different Methods of Anaerobic Threshold Evaluation in Heart Failure Prognostic Assessment
2022, Chest
In clinical practice, anaerobic threshold (AT) is used to guide training and rehabilitation programs, to define risk of major thoracic or abdominal surgery, and to assess prognosis in heart failure (HF). AT of oxygen uptake ( $\dot{V}$ O₂; $\dot{V}$ O₂AT) has been reported as an absolute value ( $\dot{V}$ O₂ATabs), as a percentage of predicted peak $\dot{V}$ O₂ ( $\dot{V}$ O₂AT%peak_pred), or as a percentage of observed peak $\dot{V}$ O₂ ( $\dot{V}$ O₂AT%peak_obs). A direct comparison of the prognostic power among these different ways to report AT is missing.
What is the prognostic power of these different ways to report AT?
In this observational cohort study, we screened data of 7,746 patients with HF with a history of reduced ejection fraction (< 40%) recruited between 1998 and 2020 and enrolled in the Metabolic Exercise Combined With Cardiac and Kidney Indexes register. All patients underwent a maximum cardiopulmonary exercise test, executed using a ramp protocol on an electronically braked cycle ergometer.
This study considered 6,157 patients with HF with identified AT. Follow-up was median, 4.2 years (25th-75th percentiles, 1.9-5.0 years). Both $\dot{V}$ O₂ATabs (mean ± SD, 823 ± 305 mL/min) and $\dot{V}$ O₂AT%peak_pred (mean ± SD, 39.6 ± 13.9%), but not $\dot{V}$ O₂AT%peak_obs (mean ± SD, 69.2 ± 17.7%), well stratified the population regarding prognosis (composite end point: cardiovascular death, urgent heart transplant, or left ventricular assist device). Comparing area under the receiver operating characteristic curve (AUC) values, $\dot{V}$ O₂ATabs (0.680) and $\dot{V}$ O₂AT%peak_pred (0.688) performed similarly, whereas $\dot{V}$ O₂AT%peak_obs (0.538) was significantly weaker (P < .001). Moreover, the $\dot{V}$ O₂AT%peak_pred AUC value was the only one performing as well as the AUC based on peak $\dot{V}$ O₂ (0.710), with an even a higher AUC (0.637 vs 0.618, respectively) in the group with severe HF (peak $\dot{V}$ O₂ < 12 mL/min/kg). Finally, the combination of $\dot{V}$ O₂AT%peak_pred with peak $\dot{V}$ O₂ and $\dot{V}$ per CO₂ production shows the highest prognostic power.
In HF, $\dot{V}$ O₂AT%peak_pred is the best way to report $\dot{V}$ O₂ at AT in relationship to prognosis, with a prognostic power comparable to that of peak $\dot{V}$ O₂ and, remarkably, in patients with severe HF.
Features of electromyography threshold of the respiratory muscles during incremental exercise test
2022, Respiratory Physiology and Neurobiology
Citation Excerpt :
An increase in lactate concentration occurs during the incremental exercise test due to anaerobic metabolism added to aerobic metabolism that exceeds VT. The process of lactate production produces H+ and decreases pH; however, for about 2 min after exceeding VT, it is first buffered by HCO3− in the body (Wasserman et al., 2017; Whipp et al., 1989). The period of isocapnic buffering is set up as a gradual increase in ventilation due to CO2 production associated with buffering of HCO3−; after this period, the progression of metabolic acidosis provokes hyperventilation.
In this study, we aimed to examine the electromyography threshold (EMGT) of the respiratory muscle and accessory respiratory muscles. Sixteen healthy men underwent an incremental exercise test at 15 W/minute to the end point. Expired gases and electromyograms of the respiratory and lower limb muscles were measured. The breakpoints for the EMG and expired gas data were analyzed using a segmented regression model. EMGT of the sternocleidomastoid and diaphragm was significantly more delayed than the ventilation threshold (VT) (287.94 s, 288.15 s vs. 185.5 s, p = 0.028 and 0.044, respectively). The EMGT of respiratory muscles and VT were not related, though EMGT of rectus femoris and vastus lateralis correlated with VT (r = 0.854, p < 0.001; r = 0.657, p = 0.011, respectively). EMGT of respiratory muscles may be influenced by multiple factors, such as central command and afferent input of mechanical stimulation from muscles, in addition to VT-induced changes in metabolic dynamics.
Evaluation of salivary α-amylase and immunoglobulin A responses after endurance exercise in adolescent males and females with similar aerobic fitness
2024, European Journal of Oral Sciences
Quantifying Improvement in V·O<inf>2peak</inf> and Exercise Thresholds in Cardiovascular Disease Using Reliable Change Indices
2024, Journal of Cardiopulmonary Rehabilitation and Prevention

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^∗: Present address: Laboratory of Applied Physiology, California State University, Long Beach, CA 90840, U.S.A.

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Ventilatory control of the ‘isocapnic buffering’ region in rapidly-incremental exercise

Abstract

Respir. Physiol.

Clin. Chim. Acta

Rate of change of alveolar carbon dioxide and the control of ventilation during exercise

J. Physiol. (London)

Muscular exercise

Breath-by-breath measurement of true alveolar gas exchange

J. Appl. Physiol.

Alveolar ventilation: recent model analysis

The effect of increased lung volume on the expiratory rate of rise of alveolar carbon dioxide tension in normal man

J. Physiol. (London)

Respiratory patterns and control during unrestrained human running

Influence of peripheral chemoreceptors on the dynamics of the exercise hyperpnœa in man

J. Physiol. (London)

Predicted values for clinical exercise testing

Am. Rev. Respir. Dis.

Exercise testing in pulmonary evaluation: rationale, methods and the normal respiratory response to exercise

N. Engl. J. Med.

Pulmonary gas exchange dynamics and blood gas regulation in incremental exercise

J. Appl. Physiol.